Initially, optical fibers were used as single-channel devices for transmitting lightwave signals. However, demand for increased bandwidth has caused the development of techniques for using optical fiber as multi-channel devices. A pulse of a lightwave signal within a channel contains a narrow spectrum of wavelengths. An optical fiber may be such that different wavelengths of a pulse propagate at different rates. Different propagation rates cause a pulse to spread in time. The extent to which a pulse is spread determines how close the channels may be spaced during transmission and still be discernible as different pulses. The minimum spacing between channels in an optical fiber determines the maximum transmission bandwidth of the optical fiber. The spreading of a pulse is called dispersion. Dispersion may be caused by the characteristics of the fiber (i.e., the refractive index of the core and cladding, etc.). Such dispersion is called material dispersion. Dispersion may also be caused by the geometry of the fiber. This type of dispersion is called waveguide dispersion. The combination of material dispersion and waveguide dispersion is called chromatic dispersion and is the primary limiting factor on transmission bandwidth. Dispersion of a waveguide is often characterized as a curve on a graph, where the x-axis of the graph is in units of wavelength (e.g., nm), and where the y-axis is in units of magnitude (e.g., psec/nm/km). A dispersion curve may be such that each wavelength maps to the same magnitude of dispersion (i.e., a flat, or zero slope, curve) or each wavelength maps to a different dispersion magnitude. If each wavelength maps to a different dispersion magnitude then the curve may either slope upward from left to right (i.e., positive dispersion slope) or downward from left to right (i.e., negative dispersion slope).
If the dispersion magnitude of a waveguide is zero for every wavelength then no dispersion, or delay, will be added to any of the wavelengths of the optical signal propogating in the waveguide and the optical signal will emerge from the waveguide undistorted. Such an optical fiber is called a zero dispersion fiber. Such fibers are more expensive than other types of optical fiber (e.g., single-mode fiber). Also, it may be prohibitively expensive to replace existing non-zero dispersion fiber with zero-dispersion fiber.
Optical fiber that has a dispersion slope, either positive or negative, has one point at which dispersion magnitude is zero. This zero dispersion point is around 1310 nm for non-zero dispersion fiber. For the most inexpensive and ubiquitous optical fiber (i.e., single-mode fiber) the dispersion slope is positive.
If the dispersion slope of a waveguide is positive then each wavelength propagating in the waveguide will have a different amount of delay added thereto, where the amount of delay added is proportional to the wavelength. That is, a smaller amount of delay is added to the shorter of two wavelengths than to the longer of two wavelengths. Therefore, the longer of two wavelengths will propagate slower through the waveguide than the shorter wavelength. The signal emerging from the waveguide will be distorted (i.e., broadened in time) with respect to the signal that entered the waveguide. The longer of the two wavelength components will lag the shorter wavelength component. Since an optical fiber has a fixed upper limit on bandwidth, a broader signal takes up more bandwidth and, therefore, lowers the number of usable channels within the transmission bandwidth of a waveguide. The optical communication industry is always looking for ways to increase the transmission bandwidth of a waveguide.
If the dispersion slope of a waveguide is negative then each wavelength will have a different amount of delay added thereto, where the amount of delay added is inversely proportional to the wavelength. That is, a larger amount of delay is added to the shorter of two wavelengths than is added to the longer wavelength. Therefore, the shorter of the two wavelengths will propagate slower through the waveguide than the longer wavelength. The signal emerging from the waveguide will be distorted (i.e., broadened in time, but in an opposite sense than for positive dispersion) with respect to the signal that entered the waveguide. The shorter of the two wavelengths will lag the longer wavelength. This lowers the number of channels within the usable transmission bandwidth of a waveguide.
To optimize the transmission bandwidth of an optical signal through a waveguide, dispersion must be compensated for periodically. One device for compensating for dispersion includes a length of fiber with dispersion equal to a single-mode fiber but opposite in polarity. Such a fiber is called a dispersion compensated fiber (DCF). However, DCF may introduce non-linearities when used with single-mode fiber because DCF has a smaller core diameter than does a single-mode fiber. In practice, the dispersion of a DCF may not exactly cancel out the dispersion of a single-mode over a broad band of wavelengths. Also, there are higher losses of light in DCF than in single-mode fiber due to the smaller diameter core of the DCF.
Another method of compensating for dispersion is to use a fiber Bragg grating (FBG). A FBG is a short-period chirped grating that is photo-induced into an optical fiber. The FBG disperses wavelengths within one channel of a fiber by reflecting different wavelengths at different times (e.g., longer wavelengths sooner than shorter wavelengths or vice versa). By reflecting different wavelengths at different times, an equal magnitude, but opposite slope, of dispersion may be introduced into the fiber to compensate for the accumulated dispersion. However, a FBG is neither the least expensive nor the least labor intensive method of compensating for dispersion.
U.S. Pat. No. 4,750,802, entitled "OPTICAL FIBER DISPERSION COMPENSATOR"; and U.S. Pat. No. 5,473,719, entitled "OPTICAL DISPERSION COMPENSATOR," each disclose a device for compensating for dispersion that separates a dispersion-distorted optical signal into a plurality of wavelengths and then sends each of the plurality of wavelengths down a separate fiber. Each fiber is tuned to add an equal amount, but opposite polarity and opposite slope, of dispersion to a particular wavelength. The dispersion-compensated wavelengths are then recombined to recover the original undistorted optical signal. U.S. Pat. Nos. 4,750,802 and 5,473,719 each require an optical fiber for each wavelength of interest, whereas the present invention does not. Therefore, U.S. Pat. Nos. 4,750,802 and 5,473,719 would be more expensive to build than the present invention. Also, insertion losses are greater in a device that splits and recombines wavelengths as does U.S. Pat. Nos. 4,750,802 and 5,473,719 than in the present invention which does not so split and recombine wavelengths. U.S. Pat. Nos. 4,750,802 and 5,473,719 are hereby incorporated by reference into the specification of the present invention.
A single-mode optical fiber consists of a glass core, a glass cladding around the core, and a plastic buffer coating around the cladding for mechanical strength. The core is doped with materials to obtain an index of refraction slightly greater than that of the cladding. The buffer coating is fabricated to obtain an index of refraction greater than that of the cladding. Lightwaves may be transmitted within the core with little loss of light.
The term single-mode optical fiber indicates that a lightwave is transmitted in a single way, or spatial mode, through the core of the optical fiber. With the buffer coating in place, any lightwave that escapes the core due to bending, or other means, enters the cladding and is absorbed by, and lost in, the buffer coating. If the buffer coating is removed, lightwaves may enter the cladding and may propagate in a number of ways, or spatial modes, through the uncoated cladding. The greater the angle of the lightwave entering the fiber the greater number of reflections per unit length the lightwave will be subjected to as it propagates in the fiber and the higher the spatial mode in which the lightwave travels. A spatial mode is characterized by the pattern created by the reflection, per unit length of fiber, of the lightwave. A lightwave in a high-order spatial mode experience a greater number of reflections, per unit length of fiber, than do lightwaves in a lower-order spatial mode. Typically, lightwaves propagate in the lowest-order spatial mode in the core of the fiber (i.e., LP(0,1)).
A device for causing lightwaves in one spatial mode, in either the core or cladding of a waveguide, to propagate in another spatial mode is called a spatial-mode converter. The geometry of the waveguide determines in which spatial mode a lightwave will propagate or, in other words, which spatial modes are supported by the waveguide. A lightwave that propagates in a waveguide may only be converted to a spatial mode that is supported by the waveguide. Examples of spatial-mode converters include gratings that are photo-induced into the waveguide, microbends physically-induced into the waveguide, and an acoustic flexural-wave device. The acoustic flexural-wave device generates an acoustic wave that flexes the waveguide and induces a grating in the waveguide. The induced grating may be used to convert a lightwave in one possible spatial mode to another possible spatial mode. The acoustic flexural-wave device includes a piezoelectric device and a glass cone attached, at its base, to the piezoelectric device. The tip of the cone is attached to the waveguide. A user-definable electrical signal is applied to the piezoelectric device to generate a user-definable acoustic flexural-wave. The frequency of the electrical signal determines in which higher-order spatial mode a wavelength of an optical signal will be converted and, thereafter, propagate.
U.S. Pat. No. 5,185,827, entitled "APPARATUS FOR COMPENSATING CHROMATIC DISPERSION IN OPTICAL FIBERS"; U.S. Pat. No. 5,261,016, entitled "CHROMATIC DISPERSION COMPENSATED OPTICAL FIBER COMMUNICATION SYSTEM"; and U.S. Pat. No. 5,371,815, entitled "LOW-LOSS DUAL-MODE OPTICAL FIBER COMPENSATORS"; and an article by Craig D. Poole et al, entitled "Optical Fiber-Based Dispersion Compensation Using Higher Order Modes Near Cutoff," published by the IEEE in the Journal of Lightwave Technology, Vol. 12, No. 10, October 1994, pp. 1746-1758, disclose the use of two-mode, or multi-mode, optical fiber to compensate for dispersion, that higher amounts of dispersion at either polarity may be generated by using high-order spatial modes near the cutoff wavelength of the optical fiber, and that a polarization controller and a polarization rotator may be used with a dispersion compensator. A two-mode fiber has one core that supports two spatial modes in the core, whereas a single-mode fiber has one core which only supports one spatial mode in the core. A multi-mode fiber has one core that supports multiple spatial modes within the core. The cutoff wavelength of an optical fiber is the wavelength below which a wavelength may propagate in a supported spatial mode in the fiber and above which a wavelength may not. The devices of these patents and article use two-mode, or multi-mode, optical fiber which is more expensive than single-mode fiber. Also, the devices of these patents and article use only the spatial modes of the core of the optical fiber whereas the present invention is not so limited. Furthermore, the devices of these patents and article introduce insertion losses when coupled to a single-mode fiber due to the coupling mismatch between these two types of fibers. Still further, the difference in the index of refraction between the core and the cladding of the devices of these patents and article is 2%. Such a difference in the index of refraction is much lower that the index of refraction generated in the present invention. Therefore, the devices of these patents and will use a greater length of fiber than would the present invention to generate an equal amount of dispersion magnitude. U.S. Pat. Nos. 5,185,827; 5,261,016; and 5,371,815 are hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,448,674, entitled "ARTICLE COMPRISING A DISPERSION-COMPENSATING OPTICAL WAVEGUIDE," discloses a device for compensating for dispersion that uses a length of dispersion compensated fiber and spatial modes of a two-mode, or multi-mode, fiber. The present invention does not use two-mode or multi-mode fiber. U.S. Pat. No. 5,448,674 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,862,287, entitled "APPARATUS AND METHOD FOR DELIVERY OF DISPERSION COMPENSATED ULTRASHORT OPTICAL PULSES WITH HIGH PEAK POWER," discloses a device for compensating for dispersion over a single-mode fiber that requires an ultrashort pulsed laser source, a pulse stretcher, and a pulse compressor. The device of U.S. Pat. No. 5,862,287 pre-compensates for the anticipated dispersion of the single-mode fiber. The dispersion introduced by the single-mode fiber compresses the stretched signal to recover the original non-dispersed signal. The present invention does not require an ultrashort pulsed laser source. U.S. Pat. No. 5,862,287 is hereby incorporated by reference into the specification of the present invention.
In an article by Timothy E. Dimmick et al., entitled "Narrow-band acousto-optic tunable filter fabrication from highly uniform tapered optical fiber," published in the Proceedings of the Optical Fiber Conference 2000, held Mar. 5-10, 2000, in Baltimore, Md., U.S.A., pp. FB4-2 through FB4-4, it is disclosed that high-order spatial modes may be created in a tapered fiber. However, the article does not disclose the present invention.